Corresponding apparatuses for ascertaining and monitoring fill level in a container are frequently utilized in measuring devices of automation- and process-control-technology. For example, measuring devices are produced and sold by the assignee under the marks Levelflex and Micropilot, which work on the basis of the travel-time measuring method and serve for determining and/or monitoring a fill level of a medium in a container. In the guided microwave method, or the time-domain reflectometry or TDR measuring method (Time Domain Reflection), a high-frequency pulse is transmitted along a Sommerfeld or Goubau waveguide or along a coaxial waveguide, and is partially reflected back at a jump in the dielectric constant (the so-called DK-value) of the medium surrounding the waveguide. Additionally, in the freely radiating, travel-time measuring method, for example, microwaves are transmitted via a horn antenna into a free space, or process space, and the echo waves reflected on the surface of the medium are received back by the horn antenna following a distance-dependent travel time of the measuring signal. On the basis of the time period between the transmitting of the high-frequency pulses and the receipt of the reflected echo signals, the distance from the measuring device to the surface of the medium can be ascertained. Taking into consideration the geometry of the container interior, then the fill level of the medium is ascertained as a relative or absolute quantity. The travel-time measuring method can be divided into essentially two methods of ascertainment: The first method of ascertainment rests on a travel-time measurement, which a pulse sequence modulated signal requires for the traveled path; a second widely distributed method of ascertainment rests on determining the frequency difference between the currently transmitted, continuously frequency-modulated, high-frequency signal and the received, reflected, high-frequency signal (FMCW—Frequency-Modulated Continuous Wave). In general, in the following, no limitation to a certain method of ascertainment is intended.
These measuring devices of automation- and process-control-technology for ascertaining fill level are often utilized in processes with aggressive media. In order to protect the sensor units (such as e.g. a waveguide, a horn antenna or an array antenna) of the measuring devices from the high-frequency-technical, thermal and chemical influences of the medium, the sensor units are protected from the aggressive media by protective elements, such as e.g. a radom, or a filler body, of a resistant, dielectric material. The reason for the protection of the sensor unit by such protective elements is, on the one hand, to prevent the corrosion of parts of the sensor unit by the medium, and, on the other hand, to prevent the formation of solid accretions and condensate, for example, in the hollow spaces of a freely radiating antenna or in cavities of a coupling unit of the waveguide. The formation of solids accretions and condensate in the cavities of freely radiating antennas and waveguides, referred to generically as accretion formation, has a direct influence on the propagation characteristic and reflection behavior of the high-frequency measuring signals. Through the accretion formation, disturbance signals occur in the measurement signal, and such disturbance signals can cover the reflection signal of the fill level, making the measuring device no longer suitable for fill-level ascertainment. In order prevent accretion in these measurements-technically, highly sensitive areas of the sensor unit, such are completely filled by a microwave transmissive, dielectric material.
A horn antenna completely filled with a dielectric material for improving durability against high-frequency-technical, thermal and chemical influences of the medium is disclosed in the following patent documents.
In German Patent DE 100 40 943 A1, a horn antenna for fill-level measurement is presented, which is filled, at least partially, with a dielectric material.
In German Patent DE 100 57 441 A1, a horn antenna for a radar-device is disclosed, whose antenna is at least partially filled with a filling of a dielectric material filled and/or the entire horn antenna is filled and completely surrounded with a dielectric material. Furthermore, the filling is so embodied on the process-side, that it forms a flange plating as a sealing element.
Furthermore, waveguide coupling units at least partially filled with a filling body of a dielectric material are known from the following patent documents.
In German Patent DE 100 19 129 A1, an embodiment of a coupling unit filled with a dielectric material is disclosed, which is able, largely, to eliminate the influence, which a structural part and/or an accretion formation on the sensor have/has on the accuracy of measurement and measuring sensitivity of the sensor. This is achieved by the elongation of the dielectric filling body of the coupling unit, whereby the structural parts lie outside of the region, into which the electromagnetic waves are radiated.
In European Patent EP 1 069 649 A1, another embodiment of a waveguide with a simple construction is disclosed, which combines the advantages of a single-wire- and a multi-wire-waveguide, in that it shows no interaction with installed objects of the container, and is cleaned of accretions or deposits in simple manner. This was achieved by surrounding the multi-wire-waveguide in the process at least partially with a dielectric medium, whereby no accretion can form between the individual waveguides.
Disadvantageous in the case of all forms of embodiment of the protective elements of the sensor units in the state of the art is that the electromagnetic waves of a high-frequency measuring signal are strongly influenced by the dielectric material of the protective element.